Both in vitro and in vivo exposure of susceptible tumor-cells to NSC 286193D results in an inhibition of the conversion of IMP to GMP. The state of quanine nucleotide starvation so produced arrests the synthesis of DNA and RNA and leads to the death of P388 and Lewis lung cells. Although the locus of action of this nucleoside appears to be at the level of IMP dehydrogenase, there was no correlation between the specific activity of this enzyme and the susceptibility of murine or human tumor cells to cytotoxicity from the drug.